Technically, it's more likely that it's /bin/sh - whatever that is. Check perl -MConfig -le 'print $Config{sh}'. On Linux, this is usually bash, but as I recall, bash does some things slightly differently when invoked as sh than as bash, largely for backwards compatability with the Bourne Shell upon which it was based.

Making /bin/sh a link to csh seems just whacked though. Most of your system won't work properly if you try it.